Abstract
In a DC plasma spray torch, the dynamic behaviour of the arc attachment at the anode nozzle results in fluctuations of arc voltage and the resulting plasma jet instabilities affect the treatment of the particles injected in the plasma flow, and thus, the coating quality. However it is not clear if the experimentally-observed fluctuations of particle temperatures are a major phenomena and if their frequencies are always in unison with those of voltage. In this study, two on-line techniques are used to investigate respectively the time-variation of particle temperatures and their correlations with voltage variations; the first technique makes it possible to analyse plasma voltage instabilities and the second one to investigate the instabilities of particle temperatures. Both allow determining the frequencies and amplitude variations of voltage and particle temperatures. The experiments are carried out with two plasma torches (F4-type and 3MB-type) using respectively argon-hydrogen or nitrogen–hydrogen mixtures as plasma-forming gases. A good correlation between arc voltage and particle temperature fluctuations is observed when the plasma torch is operating with argon-hydrogen while that's not the case when the torch is operating with nitrogen-hydrogen.
